In a typical thermal head, a plurality of heating elements or heating resistors connected in parallel to a power source are disposed to form a line of dots on a substrate. The number of heating resistors corresponds to the number of dots included in one printing line. When a voltage is applied to one of the heating resistors, the heating resistor generates heat to destroy a thermosensitive layer of a printing tape so that a colored layer under the thermosensitive layer is exposed externally. Various characters (letters, symbols, etc.) can be printed on the printing tape by selectively applying a voltage to all or part of the heating resistors according to the dot pattern of the characters.
FIG. 5 shows an example of an electrical circuit for driving a thermal head. Symbols R.sub.1, R.sub.2, R.sub.3, . . . , R.sub.N denote heating resistors disposed in a line on a substrate (not shown). The number N is 96, for example, which means that a single printing line consists of 96 dots. Numeral 1 denotes a power source to which the heating resistors R.sub.1 -R.sub.N are connected in parallel. Numeral 2 denotes a controller constituted using a microcomputer or microcomputers, which controls every part of a printer. Besides, the controller 2 produces printing data, or dot pattern data, for printing characters, such as letters, symbols or graphics, inputted by a user through an input device (a keyboard, for example). The printing data is sent in serial to a head driver 3. The head driver 3 selectively applies voltages to all or part of the heating resistors R.sub.1 -R.sub.N based on the printing data.
Numeral 4 denotes a strobe signal generator, which is controlled by the controller 2 to generate a strobe signal of a preset pulse width in each printing cycle. Symbols G.sub.1, G.sub.2, G.sub.3, . . . , G.sub.N denote gates, each gate having two inputs. One of the two inputs receives the strobe signal from the strobe signal generator 4 and the other input receives the printing data from the controller 2. When the strobe signal and the printing data are sent to the gate G.sub.x (1.ltoreq..times..ltoreq.N) at the same time, the output level of the gate G.sub.x turns from low to high, for example, and an electric current from the power source 1 is supplied to the corresponding heating resistor R.sub.x.
As described above, the electric power is selectively supplied only to such heating resistors that correspond to the gates that receive the printing data from the controller 2 while the strobe signal is ON. FIG. 6 is a time chart showing an example of form of signals in a printing cycle T. Symbol W denotes the pulse width of the strobe signal St, and symbol Vd denotes the driving voltage of the power source 1. As shown in FIG. 6, the voltage Vd gradually drops while the strobe signal St is ON and the electric power is supplied to the heating resistors.
The magnitude of drop in the voltage Vd depends on the dot pattern. The reason is explained as follows, referring to FIG. 7. In FIG. 7, the image to be printed consists of a block letter "H" surrounded by a thin rectangular frame. Symbols La, Lb and Lc denote printing lines corresponding to different parts of the image. It is assumed here that the number of dots included in a printing line is 96. In printing the line La, electric power is supplied to all the heating resistors R.sub.1 -R.sub.N (N=96) corresponding to all dots. In printing the line Lb, electric power is supplied to the heating resistors that correspond to, for example, 2 dots in the upper side and 2 dots in the lower side of the frame. In printing the line Lc, electric power is supplied to the heating resistors that correspond to 2 dots in the upper side of the frame, 2 dots in the lower side of the frame and 56 dots, for example, constituting the left column of "H".
The pulse width W of the strobe signal St is fixed, as shown in FIG. 6. Under such a condition, the amount of electric power required for printing one line increases as the number of dots to be printed in the line increases, because the number of heating resistors to which electric power is supplied increases. As a result, the voltage drop of the power source 1 becomes larger. In printing the lines La, Lb and Lc, voltage drop becomes larger in the order of Lb&lt;Lc&lt;La.
In the line Lb, the print is hardly thin because the amount of energy consumed for printing is small and the voltage drop is adequately small. In the line La, by contrast, voltage drop is considerably large. The print in the line La, however, is hardly thin because of the following reason. Where dots to be printed constitute a continuous line as in the line La, the thermosensitive layer of the printing tape is adequately destroyed within the line because each part of the thermosensitive layer facing one heating resistor receives heat not only from said one heating resistor but also from the neighboring ones.
As for the line Lc, the print is hardly thin in the left column of "H" because the dots constitute a continuous line there. The dots in the upper and lower sides of the frame, by contrast, are isolated from the continuous line. Therefore, the heating resistors corresponding to the dots in the upper and lower sides of the frame cannot generate an adequate amount of heat because of the voltage drop. As a result, the print of the frame is thinner in the line Lc than in the other part.
In the above-described conventional method, the pulse width W of the strobe signal St is fixed, as described above. Even under this condition, however, thin print can be prevented by using a large capacity power source. In this case, however, overheating occurs in a line where only few dots are printed, as in the line Lb of FIG. 7, which causes blurring around the print. Further, a large capacity power source is expensive and inevitably increases the production cost of the printer.
When a small capacity power source is used, not only the cost is reduced but also blurring around the print due to overheating does not occur. There is a possibility, however, that the print becomes partly thin, as described above. Even when a small capacity power source is used, the thin print can be prevented by suppressing voltage drop by, for example, reducing the number of dots in a continuous line as in the line Lc. This method, however, requires a complicated process and it is practically impossible to completely eliminate the thin print.
Publication No. H2-196668 of the Japanese Unexamined Patent Application discloses a method of driving a thermal head. In the method, printing is suspended when the voltage becomes lower than a preset value during the printing, until the driving voltage of the power source is restored adequately. By this method, however, longer time is required for printing because the printing cycle becomes longer.
In view of the above-described problems, the present invention proposes a method of driving a thermal head of a printer wherein thin print due to voltage drop of a power source is prevented without reducing the number of dots or changing the printing cycle by an interruptive suspension.